Ink jet printers have widely been used as the output apparatus of the computer. The ink jet printer ejects inks of various colors from multiple nozzles provided on a print head and creates dots on a printing medium, so as to implement printing. Bidirectional printing, that is, the technique of creating dots in both forward and backward passes of main scan, is known to enhance the printing speed in the ink jet printer.
In the ink jet printer, the ink ejection timing is adjusted with regard to respective nozzles, in order to create dots at predetermined positions. In the case of bidirectional printing, the ink ejection timing is adjusted according to the direction of main scan, such that the position of dots created in a forward pass of the main scan (hereinafter referred to as the forward dots) is coincident with the position of dots created in a backward pass of the main scan (hereinafter referred to as the backward dots). A test pattern is generally printed for the purpose of such adjustment.
FIG. 46 shows a prior art test pattern. This test pattern is used to adjust the positional misalignment of the forward dot and the backward dot in bidirectional printing. Each test pattern consists of a vertical line with the forward dots (the upper line) and a vertical line with the backward dots (the lower line). These lines are printed to partly overlap each other.
The backward dots are printed by shifting the drive timing stepwise in the order of Nos. 1, 2, 3, . . . . In the conditions of Nos. 1 and 2, the drive timing of the backward dot is earlier than the adequate timing, and the position of the backward dot is deviated rightward from the position of the forward dot. In the conditions of Nos. 4 to 7, on the other hand, the drive timing of the backward dot is behind the adequate timing, and the position of the backward dot is deviated leftward from the position of the forward dot. The condition of No. 3 is the optimum drive timing, in which the position of the forward dot is practically coincident with the position of the backward dot. The user selects the condition No. 3 to adjust the drive timing of the dot.
In the specification hereof, the terms ‘ink ejection timing’ ‘drive timing of the dot’, and ‘drive timing of the print head’ are synonymous.
The recent trend in the ink jet printer reduces the size of dots for the enhanced picture quality. With this trend, even a little misalignment of dot positions significantly affects the picture quality.
In bidirectional printing, the positional misalignment of dots significantly affects the picture quality. For example, the delay of the drive timing of the dot deviates the recording position of the forward dot leftward, while deviating the recording position of the backward dot rightward. The positional misalignment of dots in bidirectional printing is accordingly double the misalignment in unidirectional printing and remarkably damages the picture quality.
It is, however, difficult to detect the little positional misalignment in the prior art vertical line test pattern. This results in insufficient accuracy of adjustment of the dot recording position. Namely the prior art technique can not satisfy the accuracy of adjustment required in the arrangement of the reduced dot size and bidirectional printing. This problem is not restricted to the forward dot and the backward dot, but is commonly found for any dots created by the print head.